It's pretty easy to mash up these two medical dramas, since they both run on ABC, and Shonda Rimes created both of them. Practice star Addison Walsh (Kate Walsh) actually originated as an antagonist on Grey's, then softened considerably when she got her own show. But she appears back on Grey's when her old flame Mark Sloan (Eric Dane) asks her to consult on his daughter Sloan's (The Hunger Games's Leven Rambin) pregnancy. Double the drama!

Disney took the mash-up to the nth degree in 2009 when they merged three of their hit shows: Wizards of Waverly Place, The Suite Life on Deck, and Hannah Montana for three crossover episodes. Alex Russo (Selena Gomez) and family are just enjoying the cruise without showing off their powers; the Martin brothers (Dylan and Cole Sprouse) try to recover Hannah Montana tickets; and Hannah/Miley herself (Miley Cyrus) is on the same boat, suffering a string of bad luck.

It was like a bad fanfic, only real. My only question is, how did the universe not explode since Gomez was playing her Wizards character but had also appeared in the Hannah Montana universe playing Hannah's pop star rival Mikayla?

Grey's Anatomy/Private Practice

It's pretty easy to mash up these two medical dramas, since they both run on ABC, and Shonda Rimes created both of them. Practice star Addison Walsh (Kate Walsh) actually originated as an antagonist on Grey's, then softened considerably when she got her own show. But she appears back on Grey's when her old flame Mark Sloan (Eric Dane) asks her to consult on his daughter Sloan's (The Hunger Games's Leven Rambin) pregnancy. Double the drama!

Super Special Double Dare

While I can't find photographic evidence, I've had multiple people corroborate that this did happen. Nickelodeon ran a special edition of Double Dare with stars from the network's shows: Notably, Melissa Joan Hart and her Clarissa Explains It All co-star Jason Zimbler competed against kids from a Nick show that no one remembers. Clarissa and Sam, teaming up so they won't get slimed! It was awesome.

Boy Meets World/Sabrina the Teenage Witch

No Melissa Joan Hart this time -- Sabrina's cat Salem transports Cory, Shawn, Topanga, and Eric back to the 1940s, where they're thrown into WWII. Explosions blah blah amnesia blah blah Cory and Topanga get married in the dream world. And Salem wisecracks the whole time!

The Jetson Meet the Flintstones

You've got two acclaimed animated series set thousands of years apart. What's more sensible than to have the characters meet? Thanks to time travel, the jokes write themselves: Each group thinks the others are aliens; they're impressed with the type of technology that the other has created in that time period; and George Jetson "creates" a bunch of inventions that impress his Stone Age peers. The TV movie aired in 1987 and is a classic.

One Tree Hill/Life Unexpected

I'm sorry to say, this crossover was a major bust. OTH and LUX both built their musical guest stars on the characters being (respectively) music producers and radio hosts, so it made sense for everyone to unite at a music festival. But all it came out to was Haley (Bethany Joy Galeotti) and Cate (Shiri Appleby) bonding over how they got pregnant in high school, set to a few good songs. Yawn!

Must-See TV (Circa 1994)

What a great idea--NBC execs planned it so that on the same Thursday, all four of the NYC-based sitcoms in its lineup would incorporate plots about a blackout. It started with the Mad About You episode "Pandora's Box," where one of the characters actually caused the city-wide blackout. Friends' "The One with the Blackout" -- a classic -- picked it up, and even the shortlived show Madman of the People played along. Only Seinfeld refused to go with the joke.

Mad About You is also the show we have to thank for Phoebe Buffay's twin sister Ursula. Chronologically, Lisa Kudrow first played Ursula; then when she starred on Friends, a crossover brought on two Mad characters who thought Phoebe was Ursula. So, it got written into both shows' canons.